A 72-year-old man with a 5-year history of BPH presents with acute urinary retention. He was started on finasteride 6 months ago for a large prostate (50 g on ultrasound) and initially improved. However, over the past 3 weeks, he has had progressive hesitancy and weak stream. Tonight, he cannot void and has suprapubic pain. Catheterization yields 800 mL of clear urine. Post-catheterization, his serum creatinine is 1.8 mg/dL (baseline 1.0 mg/dL). What is the most likely reason for his acute decompensation?